This invention relates to an electronic relay, and more particularly to an electronic circuit for replacement of an A-type relay in a step-by-step selector and connector of a telephone exchange system. The A-relay is of the make-break type that responds to a subscriber's dial, or other pulsing device. Each pulse causes a moving contact to break from one relay contact and make another contact.
The telephone exchange system includes a mechanism for finding a free first selector for the first digit dialed, and successive free selectors and connector for the successive digits dialed. However, the present invention relates only to a circuit for replacing the A-relay that operates a selector or connector at each step in response to the pulses of a digit dialed.
The inherrent inertia of a mechanical relay is used to advantage in an A-type relay for preventing spurious pulses (short duration transient voltages) on the input conductors, called tip and ring, from affecting the make and break contact conditions. The problem with mechanical make-break relays is that they require periodic adjustment and contact replacement. It would be desirable to have a solid-state electronic circuit as a replacement for the make-break relay which will endure constant use for an indefinite time without adjustment.
Such solid-state circuits have been devised as field replacements for the A-type relay. They include a first differential operational amplifier having its inputs connected to detect the voltage difference between the ring and tip conductors of a subscriber's telephone line, or the equivalent conductors from SXS telephone switches, and a second saturating operational amplifier used as a comparator with hysteresis to compare the output of the first with a reference for controlling two electronic switches, one of which is normally on to connect its output terminal to a common (ground) terminal corresponding to the moving contact of the A-type (make-break) relay, and the other of which is normally off.
This normal condition exists while the subscribers line is idle, i.e., while the telephone is ON HOOK, and the output of the first amplifier is more negative than the reference. When the telephone is OFF HOOK, the output of the first amplifier becomes less negative than the reference, i.e., becomes positive with respect to the reference, and the second amplifier reverses state to cause the two electronic switches to reverse states, thereby connecting the make terminal to the common terminal. Thereafter, each dial pulse momentarily changes the first and second amplifiers back to their idle states, which momentarily connects the break output terminal to the common terminal.
To avoid operating the electronic output switches in response to spurious pulses, the circuit is arranged so that all voltage changes at the output of the first amplifier initially appear as common mode signals at the input of the comparator. This is accomplished by connecting a capacitor between the input terminals of the comparator. A silicon diode in parallel with this capacitor connects the output of the first amplifier to the reference voltage source that is applied to the comparator so that the reference voltage (with which the output signal of the first amplifier is compared) will track the output signal of the first amplifier within the voltage drop of the diode as that output signal changes to be less negative than the reference voltage.
How long the output of the first amplifier is permitted to appear as a common mode signal at the second amplifier is controlled by the RC time constant of the capacitor and a resistor coupling the output of the first amplifier to the capacitor. This RC time constant is selected to allow the circuit to respond to dial pulses in the output of the first amplifier, but not to spurious transients which might occur on the ring and tip conductors connected to terminals of the first amplifier. In that manner, the capacitor of the RC timing circuit makes all voltage changes at the output of the first amplifier appear as common mode signals to the comparator for a period designed to render the circuit operation immune to transients above the maximum dial pulse frequency of 15 Hz. In other words, the timing resistor is selected to control the interval required to charge the capacitor and thus control the upper limit of low frequencies to which the circuit will respond. Stated yet another way, this arrangement of a resistor and a capacitor functions as a low-pass filter to effectively shunt high frequency transients and pass to the comparator only the low-frequency dial pulses. The coupling resistor and shunt capacitor thus make the output of the comparator insensitive to transients on the input terminals of the first amplifier. In practice, the RC time constant is selected to be about 10 ms, a period long enough to cover some electrical (or mechanical) anomalies in the SXS telephone switching equipment.
A prior-art electronic A-type relay circuit thus performs as a DC circuit for transitions at frequencies between 0 and 15 Hertz, a low frequency band sufficient to include dialing pulses. However, the transitions of the prior-art relay circuit are not independently controlled at both the beginning and the end of the dial pulses, i.e., the prior art circuits do not independently control transitions from ON HOOK (idle) to OFF HOOK (busy), and from OFF HOOK to ON HOOK, which is how dial pulses are generated.
This lack of independent control over the RC time constant at the beginning of dial pulses is due to the fact that, for protection against spurious transitions that may appear as the beginning of dial pulses (OFF-HOOK to ON-HOOK transitions), the same coupling resistor is relied upon for the RC time constant, and the input amplifier is designed with a narrow low-pass filter (0 to 15 Hz). The low-pass filter causes a pronounced lengthening of the rise and fall of the dial pulse, which in turn affects the crossover time of the amplifier 10 output with respect to the reference voltage. This causes the overall timing to vary considerably between long and short telephone loops.
It would be desirable to independently control the RC time constant at the beginning of the dial pulses, i.e., at transitions from OFF HOOK to ON HOOK, as well as from ON HOOK to OFF HOOK. It would also be desirable to increase the cut-off frequency of the first amplifier to at least twice the maximum dial pulse frequency of 15 Hz to make response to dial pulses more reliable.